Role of particle size distribution on magnetization of two polydispersed samples of antiferromagnetic NiO particles is investigated. Zero field-cooled and field-cooled susceptibility as a function of temperature (10–350 K) data indicate that samples are superparamagnetic. This behavior is again confirmed from M–H loops (up to  kG) at 10 and 350 K. The magnetization as a function of applied magnetic field data at 350 K is analyzed considering a lognormal distribution in particle size. The estimated particle size distributions are compared with those determined from transmission electron micrographs. We find that the particle size distribution is overestimated if magnetic anisotropy is ignored while analyzing the magnetization data.